Apple has made an important interface change in Aperture 2.0, that you may not notice right away: the Primary Select control is gone. In its place is a new control for quickly changing to a Primary Only viewer mode. To understand exactly what this button does, let's re-cap.

In Aperture, you select images for two reasons: to determine which images will receive edits and adjustments, and to indicate which images you want to view in the Viewer pane. For example, you might select three images because you want to view them side by side, to determine which one you like best. When you figure out your pick, you'll want to give that image a rating. However, with all three images selected, if you pick a rating, all three images will be rated. You could, of course, deselect the other two first, but there's an easier way.

One of the images that you've selected has a thicker white border than the others. This image is the Primary Select, and it has special status. Click the Primary Only button in Aperture 2.0 and Aperture will deselect all of the images besides the Primary Select.

The cool thing, though, is that it will still leave them in the Viewer! This means you can quickly assign a rating, or metadata, or even perform adjustments, without loosing your multi-image view. When you're done, click the Primary Only button again to return to a normal selection.